Cable pulling is a commonly used technique whereby a pulling rope is attached to a cable or wire that is to be pulled through conduit. The pulling rope is then wound by a user around a capstan, which acts as a frictional force multiplier, and that is powered by a motor. This arrangement allows the user to exert a small force on the rope that tails off of the capstan and which is then translated into a large force, typically of several thousand pounds, which is exerted on the incoming pulling rope, providing enough force on the pulling rope and the cable to pull them through the conduit.
A frequent problem that occurs when pulling cable is that the pulling rope will begin to wrap upon itself on the capstan, which creates a condition called rope overlap or fouling, unless the user relaxes the force exerted on the rope as it tails off of the capstan. Once rope overlap or fouling occurs, the rope will cease to tail off the capstan, eliminating any further progress of the cable pull, forcing the user to approach the capstan and the pulling rope which is undesirable. Hence, there has been a need for devices and methods that facilitate the fleeting or lateral movement of the rope as it winds onto the capstan to prevent rope overlap.
U.S. Pat. No. 4,819,912 discloses one such device that accomplishes this task and its content is incorporated herein by reference in its entirety. It shows a capstan with a helical ramp and a guide arm to properly guide the wire onto the capstan and provide the necessary lateral force to insure proper fleeting of the rope, preventing rope overlap. The drawback to this design is that the geometry of the capstan is complex and an extra part in the form of an arm is needed, which increases the cost of the cable puller. Furthermore, the helical ramp reduces the land length available on the capstan for receiving the pulling rope, forcing the capstan to be longer or limiting the amount of pulling rope that can be wound onto the capstan. Also, the increased force that is created by using the ramp results in friction that reduces drive train efficiency. Finally, the force exerted on the guide arm can cause it to become loose, thereby allowing it to rotate with the capstan, which will force the user to stop the cable pull to fix the situation.
U.S. Pat. No. 6,073,917, whose content is also incorporated herein by reference in its entirety, also uses a helical ramp but allows the user to adjust its position on the capstan to allow for different angles at which the pulling rope can be wrapped onto the capstan. This technique also suffers from the same problems as the '912 patent, including increased cost, increased length of the capstan, and the reduction of drive train efficiency. It also requires the user to take the extra step of positioning the ramp properly which can be time consuming and adds another opportunity for operator error.
Other devices for improved fleeting of rope onto a capstan use rollers that are distanced away from the capstan, which creates more friction and drag when trying to pull cable. Some cable pullers do not have a separate mechanism for preventing rope overlap, but use a capstan with an inside sloped body having an angle of forty-five degrees, which provides the ideal mathematical angle for maximizing the outward and axial thrust on the previous wraps of rope to move them laterally on the capstan as the rope continues to wrap onto the capstan. The transfer function below, along with FIG. 1, shows exactly how this calculation is performed and why the ideal angle is forty-five degrees.F=T1*((1−μ^2)*Sin(2Θ))/2−μ)−T2*μ                Where:        F=Lateral force pushing rope outward        T1=Force normal to the capstan from first wrap of rope        T2=Force normal to the capstan from second wrap of rope        μ=Coefficient of friction        Θ=Angle of inside capstan sloped body        F=function (Sin 2Θ)        F is max at dF/dΘ=0        df/dΘ=Cos 2Θ        Therefore F is max @Θ=45°        
Each additional wrap of rope that winds onto the sloped body increases the force acting to push the rope outward on the capstan but if the rate of pulling the rope becomes too high or if too much force is exerted on the tail end, the rope will begin to climb up the sloped body. Consequently, this technique requires the user to pay attention to ensure that the rope does not climb to the top of the sloped body which will cause the rope to fall back over the topmost wrapping of rope and create an overlap condition.
For all of the above reasons, there still exists a need for an antifouling device or a rope overlap prevention device that is less costly, easier to manufacture, that does not increase friction or drag when pulling rope, and that requires less effort by a user when being used than previous antifouling devices. This is particularly true for cable pullers as the forces exerted on the cable or rope can range in the thousands of pounds and it is undesirable for the user to worry about rope overlap.